One of the most important things to remember when operating any kind of
digital mode, be it a soundcard mode or through a TNC, is you need to
keep RF out of the connecting lines. Also, with the 706, as with many
radios, if you leave the interface cable plugged into the back of the
rig, then your voice audio sounds terrible. Below is my solution:

This is an old 4 port computer switch box, with the db connectors
removed, and it now contains a fully isolated soundcard interface, a
selector switch (the original 25P4T switch), and a DPDT switch for
switching the 706 PTT from HF to VHF/UHF. On the rear are 2-1/8" phone
jacks for the audio lines to the computer. The comport cable for the
soundcard interface, the interface cable to the TNC, and the IC706MKIIG
13 pin cable are all attached internally. Below is a link to a full
schematic of this switch. The idea shown here is what you can do with a
little creativity. Ham radio "homebrewing" is a dying art, and this is
one way you can find futher enjoyment of our great hobby, by making
your own shack accessories. I have not figured an exact price, but I
expect if you can get a used comport switch at a swap meet, you may be
able to build this setup for under $20! WARNING: I assume NO
responsibilty for damage due to the construction and use of this
design. It is originally designed for an Icom IC-706MKIIG and an AEA
PK232MBX TNC. It works as designed very well.